In applicants parent application Ser. No. 07/790,516, waveguide holograms are disclosed, based on the use of thin substrate waveguides. These waveguides are characterized by the relationship between the width w of an incident laser beam coupled onto an optical waveguide having a thickness t. This relationship is controlled by .lambda., where .lambda. is the wavelength of the incident lightwave. Thin substrate waveguides are characterized by t&gt;&gt;.lambda., In this situation, one can avoid difficulties encountered in coupling the optical source to the thin film waveguide, and allow for convenient white light illumination. At the same time, t&lt;w, so that the illumination obtained is uniform.
The inventors have now discovered that these thin substrate waveguides can be particularly used for situations requiring controlled illumination. There are a wide variety of situations which require illumination of an object in an controlled fashion. This is particularly the case where one seeks to illuminate a spatial light modulator (SLM) or hologram. Certain requirements present major difficulties for conventional illumination systems. Initially, illustrating with traditional illumination of a transmissive object (FIG. 1a) and a reflective object (FIG. 1b), conventional illuminators require a substantial amount of space to perform the transformation from the wavefront emitted by the light source to the one required on the object. This space usually contains several optical elements which are the origin for stability problems, alignment difficulties and obstruction of other light beams that may be required in the optical system. Second, for some illumination, it is desired not to flood the object to be illuminated with light. Rather, the illuminator seeks to pattern the light so it hits only in preselected areas. This can increase illuminator efficiency if the light is redirected, instead of simply being partially blocked. Additionally, if the light source is broad band, spectral filtering may be required. As one example of such a situation, illumination of holograms presents particular problems. Some may have their own built-in spectral filters, while others permit white light illumination. If incoherent light sources used, the filter provides the needed amount of spatial coherence, while if lasers are employed, the filter is required to clean up the coherent noise. Accordingly, it remains an object of those of skill in the art to provide a method for selectively illuminating demanding objects, such as holograms and spatial light modulators.